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Astrophile: Searing hot exoplanet is an unearthly blue

By Jacob Aron

Cornflower planet

(Image: M. Kornmesser/ESA/NASA)

Zooming through interstellar space on your antimatter-powered craft, you spot what looks like a familiar blue dot. After years on the move, relief sets in, until you approach closer and notice that the dot is a deep azure, nothing like the pale blue of home. You’ve stumbled upon another blue planet.

It turns out this is a mistake future interstellar explorers could make&colon; astronomers have measured the colour of an exoplanet for the first time and it is blue – though that’s where its similarities with Earth end.

The planet, HD 189733b, was first spotted in 2005, orbiting a star 63 light years from Earth. Since then, it has become the best-studied exoplanet. “It’s the only one where we can really start describing the conditions,” says Frédéric Pont of the University of Exeter, UK.

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Those conditions are pretty alien. The planet is a hot Jupiter, a gas giant like the ones in our solar system but with a surface roasting at 844 °C thanks to its proximity to its host star (so close by that the planet’s year lasts just 2.2 days). That means that despite signs of carbon dioxide, water vapour and methane in its atmosphere, it is far too hot to host life.

Clouds of glass

HD 189733b is far too distant for us to separate light reflected from it from that emitted by its star. That makes it impossible to see the form, let alone the colour, of the planet. So Pont and colleagues deduced its hue by using the Hubble Space Telescope to look for changes in the light as HD 189733b passed behind its star.

Whenever this happened, the researchers saw a much larger dip in blue wavelengths than in any others, suggesting that the planet mostly reflects blue light.

Earth looks blue from space not due to its water, but thanks to light scattering off nitrogen and oxygen molecules in the atmosphere. HD 189733b’s colour is probably due to other substances.

Like other hot Jupiters, the planet’s atmosphere contains clouds of solid particles, including silica, which is reflective. “The most reasonable explanation is that the reflection is caused by clouds of grains of glass,” says Pont. Sodium atoms, which absorb red and green wavelengths, are likely colouring the reflections a deep blue.

Photography challenge

The planet may also have clouds that shine white against this blue backdrop, as in the picture above – or the silica may be mixed more evenly, giving a uniform colour, like that of Neptune.

The difference is that whereas HD 189733b is true blue, Neptune, like Earth, is barely blue, with an almost “flat” spectrum of reflections that is slightly skewed towards the blue due to a small amount of red-absorbing methane. “An alien astronomer would call it white,” says Pont.

Pont next wants to measure the colour of other exoplanets, as figuring out the range of possible hues could help home in on habitable worlds. “The whole idea is to put the Earth in context,” he says. In the same way that HD 189733b’s deep blue colour is a sign that it is startlingly different world to Earth, we might find an Earth-like planet with an active water cycle by looking for a pale blue dot.

Don’t expect a dark-blue-dot picture of HD 189733b any time soon. Since the planet orbits so close to its star, even an incredibly powerful space telescope would struggle to see it distinctly. “We will get pictures of exoplanets that are far from their star, but for this particular case it might be a challenge,” says Pont.

“Planet HD 189733b is blue, and there’s nothing I can do,” doesn’t roll off the tongue like the original David Bowie lyric, but at least it is scientifically accurate.